An electronic control system for scheduling gearshifts in a four speed automatic transmission includes storing in electronic memory accessible to a microprocessor, six functions relating vehicle speed to throttle position, one function being related to each upshift and downshift. Gearshifts are made by probing computer memory with current values for throttle position and vehicle speed, and determining on the basis of the stored shift schedule whether an upshift or downshift is required. Upshift points are determined also on the basis of the maximum engine speed when a wide open throttle condition is detected. Ambient barometric pressure alters the vehicle speed and engine speed attained at high altitude in comparison to those speeds that result for the same throttle position at sea level. Therefore, shift points defined by the schedule on the basis of vehicle speed and throttle position, and shift points related to wide open throttle conditions and engine speed, are corrected for altitude or barometric pressure changes from a reference barometric pressure at which the shift points are determined for the functions stored in computer memory.
Wide open throttle shift points are corrected for altitude by comparing current engine speed to the calculated engine speed resulting from the sum of a calibration constant stored in computer memory, which define the engine speed above which an upshift is to occur at sea level, and a product formed by multiplying a barometric pressure interpolation factor and a predetermined engine speed of which the shift point at sea level is reduced to account for altitude difference. This speed difference is multiplied by the barometric interpolation factor and the product is added to the sea level shift point engine speed to find the wide open throttle engine speed corrected for altitude.
The second altitude correction is based on the stored schedule of shifts relating throttle position to vehicle speed. In this case, the shift schedule is corrected by comparing current vehicle speed to an altitude corrected vehicle speed involving a calibration constant vehicle speed where an upshift is made at sea level, a barometric pressure integration factor, a calibration constant stated in terms of vehicle speed difference representing a correction of vehicle speed at sea level where the upshift is made, and the ratio of actual N/V to base N/V stored in computer memory.